The invention relates to an optical cable according to the preamble of claim 1.
Sometimes it is neither technically possible nor economically effective to use cables installed underground or in conduits. This can be the case in rural areas with a low population density, in areas with rocky subsoil and in mountainous areas. In such situations there is reason to use aerial cables.
Metal-free aerial cables have become known, these being chiefly installed in areas affected by frequent storms or electromagnetic influences. Non-metallic, self-supporting aerial cables can also be used in high-voltage networks. Such cables must withstand the heavy mechanical demands of these applications and be easily installable.
The disadvantages of metal-free aerial cables consist in that strong electric fields produced by high-voltage lines cause severe ageing of the protective, synthetic resin sheath. This ageing leads to the premature destruction of the protective sheath and thus to the destruction of the non-metallic aerial cable. Therefore their use is very limited. Further disadvantages are additional loading of the mast system and the different expansion coefficients of non-metallic aerial cables and metallic phase cables. A further disadvantage consists of the different sag behaviour under ice- and/or wind stress which likewise restricts the use of dielectric aerial cables.
Additionally, due to the tension elements which must be provided, the cross-section of the metal-free aerial cables is relatively large, so that such cables have a particularly high wind resistance.
Conventional overhead cables containing optical waveguides have become known. One embodiment consists in that one wire of the cable is replaced by a metal tube with exactly the same dimension as the wire, at least one optical waveguide being arranged in the metal tube. Such cables are used as earthing conductors or also as phase cables. They have the disadvantage that the number of optical waveguides which can be accommodated is limited.
xe2x80x9cTransfil Europexe2x80x9d No. 35, March 1996, p. 62-67 has disclosed an aerial cable composed of a circular aluminium profile into which outwardly open grooves are introduced. So-called multifibre buffer tubes, i.e. synthetic resin tubes with optical waveguides extending therein are accommodated in these grooves. A plurality of densely packed steel wires provided with an aluminium coating are arranged on the circular aluminium profile. The steel wires essentially serve as strain relief elements. Additionally they prevent the buffer tubes from becoming displaced from the grooves.
The object of the invention is to provide an aerial cable which has a small cross-section together with a large number of fibres. Furthermore the optical waveguides are to be very substantially externally protected from mechanical, chemical and thermal attack.
This object is achieved by the features described in the characterising clause of claim 1.